The action of a receptor aggregation factor (RAF) produced specifically by neurons aggregates the acetylcholine receptors (AChR) on mucles. The action of the RAF is strongly inhibited by short exposure of the test system (myotubes pluls RAF) to light. Innervatio of muscle by cliary ganglion neurons is decreased in a time dependent manner by co-culture with spinal cord neurons but not by dorsal root ganglion nerons.